x86: vdso put vars in rodata

This adds a const to the definitions vvar.c makes, so that the vdso_*
variables go into .rodata instead of .data.  This is essentially a
cosmetic change, just giving the section headers in the vDSO file more
pleasing flags. These variables are read-only from the perspective of
the vDSO itself and user mode, even though the contents of the DSO image
were adjusted at boot.

Signed-off-by: Roland McGrath <roland@redhat.com>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>